1. Field of the Invention
This invention relates to a wet flue gas desulfurization process wherein sulfur dioxide present in flue gas is absorbed into a slurry containing a calcium compound, the slurry having sulfur dioxide absorbed therein is oxidized to form a gypsum slurry, and gypsum is separated and collected therefrom conveniently and efficiently, and it also relates to a system for carrying out this process.
2. Description of the Related Art
In recent years, wet desulfurization systems of the so-called tank oxidation type have become popular. In these systems, the necessity of an oxidation tower is eliminated by supplying air to the tank of an absorption tower so that a slurry (containing a calcium compound such as limestone) having sulfur dioxide absorbed therein may be oxidized by contact with air. FIG. 8 is a schematic view illustrating an example of a wet lime-gypsum desulfurization system of this type.
This system includes an absorption tower 1 having a tank 2 defined in its lower part. This tank 2 is equipped with a stirring rod 4 supported by a hollow shaft 3 and rotated horizontally by a motor (not shown), air supply pipes 5 extending from hollow shaft 3 and having their open ends 5a below stirring rod 4, and a rotary joint 6 for connecting the proximal end of hollow shaft 3 to an air source. By rotating hollow shaft 3 while supplying air under pressure, air C is supplied from air supply pipes 5 to gas-phase areas created on the back side of the rotating stirring rod 4 and the tail ends of these gas-phase areas are subjected to a scattering phenomenon under the action of vortex forces resulting from the rotation of stirring tank 2. Thus, a large number of substantially uniform minute air bubbles are produced, enabling the air to come into efficient contact with the absorbent slurry having sulfur dioxide absorbed therein and form gypsum by complete oxidation of the absorbent slurry.
More specifically, in this system, untreated flue gas A is introduced into a flue gas inlet section 1a of absorption tower 1 and brought into contact with an absorbent slurry injected from header pipes 8 by means of a circulating pump 7 to absorb and remove sulfur dioxide present in untreated flue gas A. The resulting flue gas is discharged as treated flue gas B from a flue gas outlet section 1b. The absorbent slurry injected from header pipes 8 flows downward through a packing material 9 while absorbing sulfur dioxide, and enters tank 2 where it is oxidized due to contact with a large number of air bubbles produced by the above-described scattering phenomenon while being stirred with stirring rod 4, and then undergoes a neutralization reaction to form gypsum. The predominant reactions occurring in the course of these treatments are represented by the following reaction formulas (1) to (3).
(Reaction in the absorption tower) EQU SO.sub.2 +H.sub.2 O.fwdarw.H.sup.+ HSO.sub.3.sup.- ( 1)
(Reactions in the tank) EQU H.sup.+ +HSO.sub.3.sup.- +1/2O.sub.2 .fwdarw.2H.sup.+ +SO.sub.4.sup.2-( 2) EQU 2H.sup.+ +SO.sub.4.sup.2- +CaCO.sub.3 +H.sub.2 O.fwdarw.CaSO.sub.4.2H.sub.2 O+CO.sub.2 ( 3)
Thus, the slurry within tank 2 has suspended therein gypsum and a small amount of limestone used as absorbent. This slurry is withdrawn by means of a slurry pump 10 and fed to a thickener 11. Using a slurry pump 11a, the resulting concentrate D is fed to a solid-liquid separator 12 where it is filtered and recovered as gypsum E having a low water content (usually of about 10%). On the other hand, the supernatant liquid F from thickener 11 and the filtrate from solid-liquid separator 12 are conveyed to a filtrate tank 13, where limestone G is added and the resulting mixture is recycled to tank 2 as a part of the absorbent slurry by means of a slurry pump 14.
Moreover, in order to maintain a high degree of desulfurization and a high purity of gypsum during operation, the sulfur dioxide concentration in untreated flue gas A and the pH of the slurry within tank 2 are detected with sensors, and the feed rate of limestone and the feed rate of the absorbent slurry are suitably regulated by means of controllers (not shown). The absorbent slurry is fed from a separate limestone slurry tank (not shown).
Thus, in the conventional wet flue gas desulfurization system, solid-liquid separation equipment comprising a plurality of complicated and large-sized units such as slurry pumps 10 and 11a, thickener 11 and solid-liquid separator 12 (comprising a centrifugal separator, a belt filter, a decanter type centrifugal settler or the like) is used for the purpose of solid-liquid separation of the gypsum slurry. Moreover, filtrate tank 13 and slurry pump 14 are required to decrease the amount of waste water by reusing the separated water. Consequently, it has been strongly desired to achieve a size reduction and simplification of the equipment required for the solid-liquid separation step and thereby further reduce the size of and the installation space for the wet flue gas desulfurization system.